The NOT Gate and Bit Flips

The NOT gate is the simplest reversible operation: it flips a bit, $b \mapsto 1 - b$. Applying it twice returns the original value, so NOT is its own inverse — a property it shares with its quantum counterpart, the Pauli-X gate ($X^2 = I$).

That "apply twice to undo" idea is everywhere in quantum computing: many gates are their own inverse, and recognizing them lets you simplify circuits and design un-computation steps.

Try it

Start from the bit 1, apply NOT twice, and return the result.

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